Spad 502 instrument

The chlorophyll contents of the leaves were determined using a SPAD-502 instrument (Konica Minolta, Japan). This device determines the relative chlorophyll content using dual wavelength optical absorbance measurements (at wavelengths of 620 and 940 nm) of leaf samples. The chlorophyll content was obtained from 5 selected points on each leaf during measurement, and the average value was taken to reflect the chlorophyll content of the whole leaf.
Photosynthetic parameters such as intercellular CO₂ concentration (Ci), net photosynthetic rate (A), stomatal conductance (gs), transpiration rate (E), and water use efficiency (WUE) of maize leaves and water vapor pressure (VPD) were measured by a LI-6400 photosynthesis instrument (LI-COR, United States).
The drying method was adopted to measure the aboveground and belowground biomass of maize: the roots and shoots of maize were heated at 105°C for 30 min, dried at 75°C for 3 days, and weighed to determine the dry weight of maize. Mycorrhizal colonization was determined by the Phillips and Hayman methods (Phillips and Hayman, 1970 (link)).

At the R2 growth stage, the chlorophyll content was measured using a SPAD502 instrument (Minolta Camera Co. Ltd., Japan). The net photosynthetic rate, stomatal conductance, intercellular CO₂ concentration and transpiration rate were measured using the Lcpro+ Ultra Compact Photosynthesis System (ADC BioScientific Ltd., Herts, England). Three leaves (the third leaf above the uppermost ear, the ear leaf and the third leaf below the uppermost ear) were measured three times, and three randomly selected plants were measured in each row. The leaf angle (LA) was determined as previously described (Ku et al., 2010). Seven leaves (the three leaves above the uppermost ear, the leaf at the ear position and the three leaves below the dominant ear) were employed to measure the LA.
To avoid the marginal effect, the ears of the plants in the two middle rows except for the two plants at both row ends of each plot were harvested at the R6 growth stage and used to quantify the grain yield per plant and grain yield per plot. The grain yield per plot was normalized using the average plant grain yield for four plants in a row.

The leaf fresh weight and shoot height c were determined after 5 days of NaCl treatment, as described by Tan et al. [44 (link)]. The relative chlorophyll contents were made using a SPAD-502 instrument (Konica-Minolta, Tokyo, Japan) between 9:00 a.m. and 10:00 a.m. Moreover, for determination of Na⁺ and K⁺ contents, we dried cotton samples for 72 h in an oven at 70 °C, weighted and ground them into a fine power, and then digested them with 10 mL of a mixture of HNO₃/HClO₄ (83:17, v/v) overnight. Contents of Na⁺ and K⁺ were determined using a Hitachi Z-2000 atomic absorption spectrophotometer (Hitachi, Tokyo, Japan).